1. Field of the Invention
The present invention relates to a motor control apparatus and, more specifically, to a motor control apparatus having a function of discharging energy stored in a smoothing capacitor provided in a DC link.
2. Description of the Related Art
A motor control apparatus for driving a motor in a machine tool, a forging press, an injection molding machine, an industrial machine, a robot, or the like, uses a rectifier for converting AC power of a three-phase AC input power supply into DC power, and an inverter for converting DC power output from the rectifier into AC power for driving a motor.
Recently, due to demands for reducing power supply harmonics and reactive power, application of a rectifier (PWM rectifier) using pulse-width-modulation (PWM) has been widespread.
FIG. 1 illustrates a block diagram of a general motor control apparatus. A motor control apparatus 1000 is such that AC power supplied from an AC power supply 20 serving as a three-phase AC input power supply is converted by a PWM rectifier 110 into DC power which is then converted by an inverter 107 into AC power to drive a motor 30.
A smoothing capacitor 120 for smoothing a DC voltage is provided in a DC link on a DC output side of the PWM rectifier 110.
In addition, on an AC power supply side of the PWM rectifier 110, an LCL filter 104 is connected between the PWM rectifier 110 and the AC power supply 20 in order to prevent high-frequency signals generated by ON/OFF of semiconductor switches from flowing out to the side of the AC power supply 20.
The LCL filter 104 has damping resistors Ru, Rv, and Rw, and capacitors Cu, Cv, and Cw provided so that each of the damping resistors Ru, Rv, and Rw is series-connected to an associated one of the capacitors Cu, Cv, and Cw, and also has first inductances Lau, Lav, and Law, and second inductances Lbu, Lbv, and Lbw provided so that each first inductance Lau, Lav, or Law, and an associated one of second inductances Lbu, Lbv, and Lbw are provided at one terminal of an associated one of the resistors Ru, Rv, and Rw.
In addition, a power disconnection unit 105 is provided between the AC power supply 20 and the LCL filter 104. The power disconnection unit 105 performs connection/disconnection between the AC power supply 20 and the LCL filter 104.
In such a motor control apparatus 1000, a PWM rectifier control signal is generated from a power-supply voltage, a power-supply current, and a DC voltage. According to the PWM rectifier control signal, each semiconductor switch of the PWM rectifier 110 is appropriately turned on/off. Thus electric power having a power factor of 1 can be generated. In addition, a DC voltage obtained as an output of the PWM rectifier 110 can be maintained at a desired value. However, in principle, it is necessary to set the DC voltage output from the PWM rectifier 110 to be equal to or higher than a peak value of the AC power supply 20. Thus, the DC voltage is boosted.
Accordingly, even in a case where the motor control apparatus 1000 is stopped, and where the motor control apparatus 1000 and the AC power supply 20 are disconnected from each other by opening the power disconnection unit 105, energy is not discharged from and remains in the smoothing capacitor 120. Thus, the DC link on the DC output side of the PWM rectifier 110 becomes at high voltage. Therefore, when maintenance, such as checking/component-replacement, of the motor control apparatus 1000 is carried out, it is necessary to wait until energy remaining in the smoothing capacitor 120 is discharged by natural electric discharge. Accordingly, work efficiency is poor.
Thus, a method has been reported (see, e.g., Japanese Patent No. 5340476 (JP-B-5340476) (hereinafter referred to as Patent Literature 1)), which discharges energy remaining in a DC link on a DC output side of a PWM rectifier by providing a discharge resistor and a switch in the DC link. FIG. 2 illustrates a block diagram of a motor control apparatus according to Patent Literature 1. A motor control apparatus 2000 according to Patent Literature 1 has a discharge resistor R and a switch SW provided in a DC link on a DC output side of a PWM rectifier 110. After stopping a normal operation of the motor control apparatus 2000, and disconnecting an AC power supply 20 and the motor control apparatus 2000 from each other by a power disconnection unit 105, the switch SW is turned on. Thus, electric current i flows in the discharge resistor R, and energy stored in a smoothing capacitor 120 is discharged. According to this related art, energy remaining in the smoothing capacitor 120 can be discharged in a short time. Consequently, maintainability is enhanced. However, it is necessary that the discharge resistor and the switch are provided in the motor control apparatus. Thus, the size of the motor control apparatus is large, and the cost thereof is increased.
In addition, another method is known, which discharges energy remaining in a smoothing capacitor by applying electric current to a motor connected to an inverter (see, e.g., Japanese Laid-open Patent Publication No. 8-182400 (JP-A-8-182400) (hereinafter referred to as Patent Literature 2)). FIG. 3 illustrates a block diagram of a motor control apparatus according to Patent Literature 2. A motor control apparatus 3000 according to Patent Literature 2 is such that electric current is applied to a motor 30 by controlling an inverter 107 after stopping a normal operation of the motor control apparatus 3000, and disconnecting an AC power supply 20 and the motor control apparatus 3000 from each other by a power disconnection unit 105, and that energy stored in a smoothing capacitor 120 is discharged by a resistance component of a motor winding. The related art motor control apparatus 3000 can discharge energy remaining in the smoothing capacitor 120 without increasing cost. However, the motor 30 may involuntarily be rotated by applying electric current to the motor 30. Thus, a dangerous state may occur.